This invention relates to systems for sending bits of information from a data source over a band-limited channel by modulating a carrier in accordance with a sequence of signal points drawn from a constellation of available signal points on the basis of groups of the information bits.
Csajka et all., U.S. Pat. No. 4,077,021, and in Ungerboeck, "Channel Coding with Multilevel/Phase Signals", IEEE Transactions on Information Theory, Vol. IT-28, No. 1, January, 1982, incorporated herein by reference, disclose systems in which each group of information bits is expanded by adding one bit of coding data (used at the receiver to aid in the decoding process) and each expanded group is encoded as one modulation signal point. All groups have the same number of information bits, and the encoding is arranged so that signal points on the constellation are equally likely to be drawn.
Forney, U.S. patent application Ser. No. 439,740 (filed Oct. 8, 1982), discloses systems in which the coding data is added to only some of the groups, and the constellation is arranged so that signal points closer to the origin have a greater probability of being drawn than signal points farther from the origin.
Canadian patent application Ser. No. 352,207 (filed Feb. 25, 1982), discloses a system in which the information bits for each group are accumulated both from primary and secondary data sources. Different groups all include the same number of primary source bits but may or may not have an additional bit from the secondary source. Groups having fewer total bits are encoded as signal points nearer to the origin; groups with more total bits are encoded as signal points farther from the origin. Points nearer the origin are more likely to be selected than points farther from the origin.
Modems typically are fed input bits from a single synchronous primary data source (or multiplexed group of data sources), and in some cases also draw further bits from secondary data sources, including asynchronous sources of control bits.